One limitation of electromagnetic machines utilizing permanent magnets is that the permanent magnets provide a constant level of magnetic flux that does not necessarily correspond to the most desirable flux levels for the anticipated operating conditions of the machine. This limitation is of particular significance in applications where the electro-magnetic machine is likely to operate in significantly different operating modes. For example, the constant magnet flux limitation is of particular significance in laundry applications where the motors used in such applications are anticipated to operate in a high speed/low torque operating mode (e.g., during the spin cycle of the laundry machine) and in a low speed/high torque operating mode.
In the past and in an effort to minimize the negative consequences of the constant flux limitations described above, a number of compromises have been made. For example, in applications like laundry applications where a motor is anticipated to operate in a high speed/low torque mode, the motor is often designed to have a minimal number of winding turns in each phase winding. While the use of such a minimal number of turns tends to ensure desirable operating in the high speed/low torque mode, it creates problems if the motor is to be operated in a high torque/low speed mode because effective operation of the motor in such a mode requires that relatively high currents be established in the motor windings. To provide such currents, the power devices driving the motor, such as the inverter, must be sufficiently large to handle such large currents, resulting in increased inverter cost and complexity.
U.S. Pat. No. 5,530,307, which is incorporated herein by reference in its entirety, discloses a solution for dynamically adjusting the flux in a brushless permanent magnet dynamoelectric machine such that the motor can effectively operate in different operating modes and such that the cost and complexity of the inverter driving the machine can be optimized. The disclosed solution allows the phase switching of the machine to be accomplished with an inverter of conventional design.
In FIG. 1 of the present disclosure, a component 10 of a brushless, permanent magnet dynamoelectric machine as disclosed in the '307 Patent is illustrated. The component 10 includes a rotor assembly 20 and a flux controlling assembly 40. The rotor assembly 20 is positioned within a stator assembly (not shown) of the machine and includes a rotor shaft 22 having a plurality of stacked rotor laminations 24 mounted thereon. The rotor laminations 24 form a plurality of outwardly salient poles 26a, 26b. Permanent magnets 30a, 30b magnetically attach to the rotor laminations 24. The magnets 30a, 30b are elongated and bread loaf-shaped. First arcuate surfaces 32 of the magnets are magnetically attached to the laminations 24. The magnetic attachment produces a consequent rotor assembly in which each of the poles 26a, 26b formed by the rotor laminations 24 is now the same type pole. For example, all the poles 26a, 26b formed by the rotor laminations 24 may be south poles, and the north poles for the rotor assembly 20 are formed by the respective magnets 30a, 30b. 
The flux controlling assembly 40 is used to control the available flux coupled between the rotor assembly 20 and the stator assembly of the machine. The flux controlling assembly 40 includes a magnetic mounting fixture 42, a coil 44, a cage 50, and legs 52a, 52b. The magnetic mounting fixture 42 attaches to an end wall (not shown) of the machine by an attachment member 48, and the coil 44 is installed on the magnetic mounting fixture 42. Thus, the coil 44 is fitted about the rotor shaft 22 such that an air gap (not visible) is formed therebetween. The cage 50 is tubular and is disposed about the magnetic mounting fixture 42 and coil 44. Another air gap G3 is formed between the mounting fixture 42 and an inside diameter of the tubular cage 50.
The legs 52a, 52b on the cage 50 extend the length of the lamination stack of the rotor assembly 20. The legs 52a, 52b have arcuate outer surfaces 54 that face the stator assembly and have arcuate inner surfaces that magnetically attach to a second surface 34 of the permanent magnets 30a, 30b. With the legs 52a, 52b attached to the permanent magnets 30a, 30b and the magnets attached to the rotor assembly 20, the cage 50 is suspended about fixture 42 and can rotate in synchronism with the rotor shaft 22. The flux controlling assembly 40 can provide a diverted flux path that is different from the primary flux path between the rotor assembly 20 and the stator assembly. The diverted flux can either additively or operatively combine with the primary flux depending upon the direction of current flow supplied to the coil 44. In this way, the flux of the disclosed machine can be controlled by controlling current flow to the coil 44. With the flux controlling assembly 40, for example, it is possible to reduce machine torque, particularly at high speed, and to reduce current requirements for the machine.
Although the machine of the '307 Patent operates well and has several advantages over conventional designs, there is room for improvement. For example, the solid, elongated construction of the legs 52a, 52b of the cage 50 leads to iron losses and may produce undesirable cogging torques.
The present invention is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above.